Wireless devices or terminals for communication are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server, such as server providing video streaming service, via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, computers, or surf plates with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device or a server.
A cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. eNodeB (eNB), NodeB, B node, Base Transceiver Station (BTS), or AP (Access Point), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or low power base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the wireless devices within range of the base stations. The base stations and wireless devices involved in communication may also be referred to as transmitter-receiver pairs, where the respective transmitter and receiver in a pair may refer to one or several cells served by one or several base stations or a wireless device, depending on the direction of the communication. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station(s) to a wireless device. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless device to the base station(s).
Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for communication with terminals. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission in LTE is mainly controlled by the radio base station.
Dual Connectivity
3GPP have recently introduced support for Dual Connectivity (DC). This feature allows RAN to use resources from two cells served by different eNBs for supporting RAN/UE connectivity for a wide range of eNB interconnect latencies.
This is similar to the 3GPP concept of Carrier Aggregation (CA) that allows RAN to use resources from more than one cell served by one eNB for supporting connectivity between the RAN and a UE. A non 3GPP solution for CA can be generalized to allow RAN to use resources from more than one cell served by a different eNB but only if interconnect latency between eNBs are small enough to allow that.
Below some DC terms and other terms used herein are defined:
Cell Group: in dual connectivity, a group of serving cells associated with either the MeNB or the SeNB.
Dual Connectivity: mode of operation of a UE in Radio Resource Control (RRC)_CONNECTED, configured with a Master Cell Group and a Secondary Cell Group.
Master Cell Group: in dual connectivity, a group of serving cells associated with the MeNB, comprising of the PCell and optionally one or more SCells.
Master eNB: in dual connectivity, the eNB which terminates at least S1-Mobility Management Entity (MME).
MCG bearer: in dual connectivity, a bearer whose radio protocols are only located in the MeNB to use MeNB resources only.
Secondary Cell Group (SCG) bearer: in dual connectivity, a bearer whose radio protocols are only located in the SeNB to use SeNB resources.
Secondary Cell Group: in dual connectivity, a group of serving cells associated with the SeNB, comprising a PCell and optionally one or more SCells.
Secondary eNB: in dual connectivity, the eNB that is providing additional radio resources for the UE but is not the Master eNB.
Split bearer: in dual connectivity, a bearer whose radio protocols are located in both the MeNB and the SeNB to use both MeNB and SeNB resources.
X2 is also used as the interface between a MeNB and an SeNB.
Accordingly, there are two roles defined for eNBs that are involved in DC towards a UE, the MeNB and the SeNB. The MeNB is the only eNB that supports RRC connectivity towards the UE.
It is also the MeNB that initiate a setup of a SeNB connection to a UE via X2 Application Protocol (X2 AP) signaling towards SeNB and RRC signaling from MeNB towards the UE. X2 is an interface between eNBs that carry both user plan data and Control plane data.
There are two main options per data bearer, split bearer and direct SCG bearer.
Today both CA and DC are used for increasing user throughput by aggregating resources from different cells and/or carriers and by that get a combined gain. It has also been suggested to use resources from the same carrier but from different cells and by that achieve a layer 3 diversity gain and use that to improve mobility robustness, such as e.g. the concept of RRC diversity. Layer 3 is the network layer according to the Open Systems Interconnection (OSI) model.
Today there are two typical types of solutions for how an eNB can evaluate if an additional connection should be setup via a new cell or not.
In the first solution the eNB is configured to always try to setup and start using a connection from a specific secondary Cell towards the UE for transmission of user data. If the connection setup fails or is considered as useless for sending user data after setup as indicated by either Medium Access Control (MAC) layer e.g. by detection of DL HARQ nack or no or CRC faulty transmission in UL and/or, UE reported Channel Quality Indication (CQI) indicating poor DL quality and/or, Power Headroom Report (PHR) indicating lack of UE power feedback from a UE, or a UE RRC layer measurement report is triggered such as an A2 event which is when signals via the serving cell becomes worse than a threshold, then the additional carrier will either be released e.g. for the potential favor of some other carrier, or just deactivate transmission of user data on that carrier waiting for the connection to improve.
In the second solution the eNB is configured to start Layer 3 measurements first and only after a RRC measurement report as feedback that indicates that the secondary cell is good enough which is referred to as event A4 or A51, or best on that frequency and better than current serving cell which is referred to as event A36, the eNB initiates a connection setup from the new cell to the UE and start using a connection from the new secondary Cell towards the UE for transmission of user data.
The problems with the first solution is that there are signaling and some resources unnecessary wasted in SeNB and MeNB and UE before information that the intended new connection is not really useful for data transmission is received.
The problem with the second solution is that an Inter frequency measurement using measurement gap typically needs to be used, implying delays for getting a measurement report a higher drop risk and lower peak throughput and higher battery consumption, and when feedback is actually received that the new connection is useful the main part of the data may already have been sent from MeNB so there is no need for a new connection.